Ebolaviruses and marburgviruses are incompletely understood pathogens that cause severe, often fatal, illnesses in humans and non-human primates. These diseases have been known as Ebola and Marburg hemorrhagic fevers, respectively, after the most dramatic symptoms in severe cases. The names “Ebola virus disease” or Marburg virus disease” are now preferred by the World Health Organization (WHO) and some other groups.

Most species of ebolaviruses and the only known species of marburgvirus occur in Africa. Current evidence suggests that the reservoir hosts are probably bats, while other animals and people are incidental hosts. Humans seem to become infected with marburgviruses mainly in caves or mines harboring bats, while ebolavirus infections are often associated with handling tissues from infected nonhuman primates and other species. Once a virus has entered human populations, it can spread from person to person. Some epidemics have affected hundreds of people, particularly when nosocomial spread occurs from inadequate medical supplies or barrier nursing procedures, or when outbreaks are not recognized for long periods. An outbreak of unprecedented size in West Africa began in December 2013, and was first recognized in March 2014. It has been spread in some densely populated urban regions, and has affected thousands of people to date. Although the mortality rate has varied between outbreaks, some ebolaviruses or marburgviruses have killed up to 90% of those who become infected. Treatment options are limited, and with the exception of experimental treatments, consist of supportive care alone. Epizootics in gorillas and chimpanzees are equally serious, and may threaten the survival of these species in the wild. Other wild mammals including duikers also seem to be killed during outbreaks.

One species, Reston ebolavirus, has been reported outside Africa, in the Philippines and China. This virus does not seem to affect humans, although some people may seroconvert. However, it can cause fatal illness in some species of nonhuman primates. Between 1989 and 1996, Reston ebolavirus was isolated repeatedly at primate quarantine facilities in the U.S. and Italy; in all but one instance, infected monkeys had been imported from a single facility in the Philippines. The source of the virus was never found, but infected monkeys do not seem to have been exported after this facility was closed in 1997. In 2008, however, Reston ebolavirus was discovered in pigs during an unusually severe outbreak of porcine reproductive and respiratory syndrome (PRRS) in the Philippines. This virus was also found in pigs with PRRS in China. Based on experimental studies, Reston ebolavirus alone does not seem to cause any illness in pigs, although its effects during co-infections with other pathogens have not yet been evaluated. Accumulating evidence suggests that ebolaviruses or their relatives may also occur in other locations, although the clinical significance of these viruses for humans and domesticated animals is uncertain.